1. Field of the Invention
The invention relates generally to the correction of exposure variations in an optical system. More particularly, this invention relates to correcting variations introduced by one or more components, such as a reticule or pellicle, into the exposure field of the optical system.
2. Description of the Related Art
Optical systems are widely used in the microelectronics industry to manufacture semiconductor wafers by a process known as photolithography. Typically, a photolithography system comprises a light source configured to project light rays to a condenser lens. The condenser lens collimates the light rays towards a pellicle placed before (or after) a reticule. Typically, the reticule is an optically clear quartz substrate having a chrome pattern used to project an image onto a portion of a photoresist-coated wafer. The pellicle is a very thin, transparent film which seals off the reticule surface from airborne particulates and other forms of contamination. A projection lens is placed after the reticule to receive and focus the light rays onto an exposure field on the wafer.
In designing such an optical system, any one of these components may be vulnerable to manufacturing imperfections which, even if minutely small, may cause intolerable or unacceptable defects in the photoresist layer of the wafer. For example, aberrations due to defects in one or more lenses may include distortion, curvature of field, spherical aberration, and astigmatism. Moreover, distortions may be due to defects in the reticule that may be caused during manufacturing. For example, reticule defects may arise from impurities in the chrome, lack of adhesion of the chrome to the reticule, variances in ion beam used to produce chrome etching, or other similar defects. Reticule defects may cause intolerable or unacceptable variations in critical dimensions (CD""s) in the exposure field. ACD represents the width or space of critical elements in an integrated circuit (IC).
Several attempts were made in the industry to compensate for general defects in the optical system. For example, in U.S. Pat. No. 5,640,233 issued to McArthur et al., a two-plate corrector is disclosed in a stepper configuration so that an image from a reticule plane is projected to an ideal image at an object plane. Based on the premise that depth of field correction made at the reticule plane induces insignificant distortion, McArthur describes placement of the two-plate corrector at the reticule plane to correct depth of field distortions caused by the lens system. However, McArthur does not describe how to correct defects resulting from specific components, such as the reticule or pellicle.
To eliminate undesirable variances that result from defects in the reticule, some manufacturers simply replace the defective reticule with a new reticule. Other manufacturers resort to discarding wafers having intolerable CD""s caused by the defective reticule. In either case, a significant increase in manufacturing cost due to reticule defects has become unavoidable.
Therefore, there is a need in the industry to compensate for individual component defects without having to replace the component or discard any defective wafers resulting therefrom. The solution of correcting such defects should be cost-effective and easy to implement.
To overcome the above-mentioned limitations, the invention provides a photolithography system having a light source configured to project light onto an object. The system comprises a reticule positioned to receive the projected light from the light source, wherein the reticule includes a defect. The system further comprises a filter positioned between the light source and the reticule. The filter has a corrective element that is geometrically related to the location of and configured to substantially correct the defect in the reticule. In another embodiment, the photolithography system comprises a reticule positioned at a reticule distance from the light source to receive light therefrom, wherein the reticule includes a defect. The system further comprises a filter positioned at a filter distance from the light source that is greater than the reticule distance. The filter is configured to receive light passing through the reticule. The filter has a corrective element that is geometrically related to the location of and configured to substantially correct the defect in the reticule.
In another embodiment, the invention provides a reticule configured for use in a photolithography system. The reticule comprises a first plate having a predetermined pattern which includes a defect for projection onto an exposure field. The reticule further comprises a second plate attached to the first plate. The second plate comprises a filter having a corrective element that is geometrically related to the location of and configured to substantially correct the defect in the first plate. The invention also provides a method of correcting a defect in the reticule of the photolithography system. The method comprises the step of obtaining at least one measurement of a feature in an exposed image. The method further comprises the step of analyzing the measurement in connection with an expected feature in the exposed image. The method further comprises the step of determining the optical characteristics of a filter based on the analyzing step, the filter being suitable for substantially correcting the defect in the reticule.